1. Field of the Invention
The present invention relates to a keyless entry system that performs communication between a vehicle-side device and a portable device to lock and unlock a door, etc., and more specifically, to a keyless entry system that can detect that a portable device is left in a vehicle, using human body communication.
2. Description of the Related Art
In a keyless entry system, a vehicle-side device is provided in a vehicle, such as an automobile, to perform wireless communication with a portable device carried by a driver, and if authentication is established, a door is locked and unlocked. A keyless entry system in which, if a portable device is in a vehicle, a predetermined switch is operated to perform authentication between a vehicle-side device and the portable device, and if authentication is established, an engine is started, is also known. In such keyless entry systems, a keyless entry system using communication via a human body between the portable device and the vehicle-side device is conventionally proposed. As such a keyless entry system, for example, there is a system as described in Japanese Patent Publication No. 2002-525702.
The conventional keyless entry system has a system that determines whether the portable device is in the vehicle, or is out of the vehicle in order to prevent the portable device from being left in the vehicle. For this purpose, a plurality of antennas is installed inside and outside the vehicle, and whether or not the portable device has received a signal from any one of the antennas is determined so that whether the portable device is either inside or outside the vehicle may be determined. However, a plurality of antennas is needed for determination of whether the portable vehicle is either inside or outside the vehicle, and the determination technique is also troublesome. Further, a technique using the communication via a human body between a vehicle-side device and a portable device in order to determine whether the portable device is either inside or outside the vehicle has not been known until now.